The primary aim of the proposed project is to determine the efficacy of a single injection of 90-140 mg (proportioned to body weight) of hydrocortisone (Hcort) to prevent the subsequent development of posttraumatic stress disorder (PTSD) by engaging biological mechanisms associated with recovery within 6 hours after exposure to a traumatic event. This project also seeks to examine biological and psychological alterations associated with trauma exposure and recovery by evaluating these over time in persons randomized to Hcort or placebo. The proposed approach is an outgrowth of work performed by the investigators in their respective research settings pertaining to mechanisms involved in recovery. PUBLIC HEALTH RELEVANCE: This study, if proven effective, has the potential of revolutionizing the treatment of PTSD in the same way that administration of thrombolytic agents in the "golden hours" has changed the treatment in CVA and MI, and will move the treatment of PTSD from treatment to secondary prevention.